1. Field of the invention
This invention relates to an inverter including a switching circuit for sequentially energizing a plurality of windings of a brushless motor driving a compressor of a heat pump, at a commutation time corresponding to a predetermined rotational position of a rotor of the motor and further relates to an air conditioner controlled by such an inverter as mentioned above.
2. Description of the Prior Art
A heat pump has recently been incorporated in air conditioners. In such heat pump type air conditioners, an outdoor heat exchanger composing an outdoor equipment serves as a condenser in a cooling operation and as an evaporator in a warming operation. When an outdoor temperature drops during the warming operation particularly in winter, frost forms on the outdoor equipment of the air conditioner, which lowers its warming performance.
In view of the above-described drawback, a defrosting operation for removing the frost on the outdoor heat exchanger is executed during the warming operation. Two manners of defrosting the outdoor heat exchanger are known in the art. In one manner, the heat pump is switched to a cooling cycle by a four way valve so that the outdoor heat exchanger serves as the condenser. In the other manner, the four way valve is maintained in the warming cycle and a bypass is opened for directly supplying, to the outdoor heat exchanger, refrigerant discharged from a compressor usually comprising a casing hermetically enclosing a compressing section and an electric motor driving the compressing section. In each of the above-mentioned defrosting manners, the high temperature refrigerant discharged from the compressor is directly supplied to the outdoor heat exchanger to raise its temperature such that it is defrosted.
Generally, the necessity of the defrosting operation or detection of the frost on the outdoor heat exchanger is based on the fact that the temperature of the outdoor heat exchanger has been decreased to a predetermined value or below. Other detecting manners are also known in the art. For example, the detection of the frost is based on temperature changes of the indoor heat exchanger, changes in the difference between the temperature of the indoor heat exchanger and the room temperature, changes in the difference between the temperature of the outdoor heat exchanger and the outdoor temperature, or a rate of each of these temperature changes.
The warming performance for the interior of a room is lowered or reduced to zero during the defrosting operation. Accordingly, it is desirable that the defrosting operation be completed in a period of time as short as possible for prevention of drop of the temperature in the room. For the purpose of reduction in the period of the defrosting operation, the prior art has provided utilization of heat generated by an electric motor driving the compressor composing the outdoor equipment of the air conditioner with the outdoor heat exchanger. In this case, the motor is required to generate a large amount of heat while the defrosting operation is being executed, that is, it is required to have a low efficiency or a large loss.
On the other hand, induction motors and brushless motors classified into a DC motor have recently been employed for driving the compressors of the air conditioners for the purpose of variable performance of the compressors or saving electric power consumption. These induction motors and the brushless motors are controlled by an inverter.
FIG. 10 schematically illustrates a prior art electrical circuit arrangement for control of the induction motor by the inverter. A control circuit 1 controls transistors 5 to 10 of a switching circuit 4, thereby controlling a voltage applied to each phase winding of an induction motor 3 from a DC power supply circuit 2 and its frequency. A high efficient operation can generally be achieved by controlling the voltage applied to the induction motor 3 and the frequency so that the relationship as shown in FIG. 11 is obtained.
FIG. 12 schematically illustrates a prior art electrical circuit arrangement for control of the brushless motor by the inverter. A control circuit 11 controls transistors 15 to 20 of a switching circuit 14 on the basis of a position detection signal generated by a position detecting circuit 13 detecting the rotational position of a rotor of the brushless motor 12. Furthermore, information about the motor speed is also obtained from the position detection signal. The speed information is compared with a speed command signal for the purpose of the motor speed control by way of a pulse width modulation system.
In the control of the induction motor by the inverter, the motor loss can be readily increased with the amount of heat generated by the same being increased when the voltage is raised relative to the frequency during the defrosting operation but not in the manner that the relationship of FIG. 11 is obtained. In the brushless motor, however, the motor loss cannot be increased even if the relationship between the frequency and voltage is controlled, and accordingly, high efficient operation is performed even during the defrosting operation. Thus, the brushless motor cannot not generate so much heat as to be used for the defrosting and accordingly, the defrosting operation needs much time. Consequently, the room temperature is decreased during the defrosting, which lowers the comfortability provided by the air conditioner.
The air conditioner also necessitates the heat generated by the compressor or compressor motor in a preparatory defrosting operation or in the case where the temperature of the compressor drops during the normal operation, as well as in the above-described defrosting operation. The preparatory defrosting operation is performed for shortening the period of the defrosting operation. In the preparatory defrosting operation, the compressor is caused to generate heat prior to start of the defrosting operation and the heat residual in the compressor is used for the defrosting operation. The preparatory defrosting operation is performed when it is expected that the defrosting will be necessary or immediately before the defrosting operation. In the prior art preparatory defrosting operation, in the condition that the heat pump is maintained in the warming cycle, an indoor fan supplying the indoor air to the indoor heat exchanger is interrupted, a pressure reducer is throttled, or the speed of the compressor motor is increased over the command value based on the room temperature. The preparatory defrosting operation is completed and the defrosting operation is initiated when the temperature of the compressor has been increased to a predetermined value or above as the result of execution of the preparatory defrosting operation or when the preparatory defrosting operation has been performed for a predetermined period of time.
The indoor fan is interrupted or the pressure reducer is throttled in the preparatory defrosting operation, as described above. Accordingly, the preparatory defrosting operation also needs to be completed in a period of time as short as possible for prevention of drop of the temperature in the room. Furthermore, the compressor is maintained at about 80.degree. C. during the normal operation. The temperature of the compressor is not increased when the compressor motor is turned on and off alternately at short intervals. In this case, the operation of the air conditioner is performed with the compressor temperature low. A lubricating oil in the compressor is caused to merge into the refrigerant when the difference between the compressor temperature and that of the heat exchanger serving as the condenser, which heat exchanger is an indoor heat exchanger in the warming operation and an outdoor heat exchanger in the cooling operation, becomes small. Consequently, the viscosity of the lubricating oil is reduced and accordingly, the lubricity of the lubricating oil is reduced. Thus, since the compressor may be damaged, the compressor temperature needs to be raised in a short period of time in this case, too.
As described above, the compressor temperature needs to be raised in the heat pump type air conditioners when the preparatory defrosting operation is executed and when the compressor temperature has been decreased during the normal operation as well as in the defrosting operation. However, when the brushless motor is provided for driving the compressor of the heat pump type air conditioner, it is difficult to drive the brushless motor so that its loss is increased and accordingly, the heat for heating the compressor cannot be obtained. Consequently, the operational comfortability provided by the air conditioner and its reliability are lowered as compared with that employing the induction motor for driving the compressor.